WASHINGTON—A GOP senator with concerns about President
Donald Trump’s
trade policy said Wednesday he would block two of the president’s nominees, saying the Trump administration hasn’t been responsive to his concerns on the issue.

Sen. Tim Scott,
a South Carolina Republican, said at a Senate Finance Committee hearing Wednesday that he would hold up nominees for two deputy U.S. trade representative posts because the head of the trade office,
Robert Lighthizer,
hadn’t returned a call or answered a trade-related question.

Aides to Messrs. Scott and Lighthizer said the two met later in the day, but Mr. Scott wasn’t ready to let the nominees go forward, saying he still had questions.

“I had a chance to address those issues, but that does not necessarily mean that I have a crystal-clear picture of how those issues will be impacted through the [Nafta] negotiations,” Mr. Scott said.

The scuffle occurs at a sensitive time for trade. Already, some GOP lawmakers have expressed concerns about Mr. Trump’s trade policies and rhetoric, and their potential effect on businesses and farms that have been able to benefit from the free trade brought about by pacts like the North American Free Trade Agreement. Nafta negotiations will resume in Montreal next week.

Mr. Scott’s objection marks the second time a Republican lawmaker frustrated by the administration’s approach to trade has held up one of his nominees.
Sen. Jeff Flake
(R., Ariz.) is holding up the appointment of the chief U.S. agricultural negotiator over concerns that a provision the U.S. wants included in a renegotiated Nafta would put up barriers to Mexican produce imported through Arizona.

While a single senator can’t hold up a nomination indefinitely, a hold can require time-consuming procedural votes to confirm nominees, slowing it down.

Mr. Scott’s unusually dramatic statement was delivered at the end of a public committee hearing, about an hour after Mr. Lighthizer met behind closed doors with some senators on the panel—and left without making any public statements.

A spokeswoman for Mr. Lighthizer said, “Apparently there was a communications breakdown in our office and Ambassador Lighthizer did not know Sen. Scott had requested a call.”

Mr. Scott didn’t say what his question concerned, but Mr. Lighthizer is currently proposing new North American rules for auto trade that could affect a
BMW AG
plant in South Carolina, as well as overseeing a trade-enforcement case that could put tariffs on washing machines made by South Korea’s
LG Electronics Inc.
and
Samsung Electronics Co.
, which has built a new plant in South Carolina.

Several GOP senators on the Finance Committee, which oversees trade policy, have criticized the Trump administration for threatening to dismantle Nafta if Mexico and Canada don’t agree to sweeping new proposals, or for taking steps they worry will lead to less agricultural export opportunities for major American crops and meat products.

After announcing the hold, Mr. Scott said his disagreement has “less to do with the Trump administration and more to do with Mr. Lighthizer.” Mr. Scott, the only African-American GOP senator, criticized Mr. Trump last year following the president’s controversial reaction to the Charlottesville, Va., protests and later met with the president on the issue.

Earlier in the day, Mr. Scott told The Wall Street Journal that he has concerns about Mr. Lighthizer’s proposals on requiring cars traded in North America to have high levels of U.S. content and on boosting the level of regional content for cars traded duty-free. These so-called “rules of origin” are one of a handful of proposals from the Trump administration that complicated talks with Mexico and Canada.

The nominees include two deputy U.S. trade representatives. One nominee,
C.J. Mahoney,
is set to oversee China and other trade issues and will also address congressional concerns about transparency, according to
Sen. Ron Wyden
(D., Ore.), who said Mr. Lighthizer confirmed Mr. Mahoney’s transparency role in the closed-door meeting during the public hearing. Mr. Wyden last week led a Democratic letter criticizing the Trump administration for not following transparency procedures and briefing lawmakers on pending trade matters.

The other nominee,
Dennis Shea,
is set to work out of Geneva, home of the World Trade Organization, the trade body the Trump administration has frequently criticized.

The chairman of the committee,
Sen. Orrin Hatch
(R., Utah), said upon leaving the hearing that he opposes Mr. Trump’s warnings about exiting Nafta.

“He knows how I feel—we’ll have to just see what happens,” Mr. Hatch told reporters.